1. Field of the Invention
The present invention relates generally to a broadband wireless access system, and more particularly, to an apparatus and a method for synchronization of a scanning cycle.
2. Description of the Related Art
Worldwide Interoperability for Microwave Access (WiMAX) provides high-speed Internet access for a mobile or stationary Mobile Station (MS). Accordingly, WiMAX systems should guarantee seamless data transmission and reception during MS mobility.
WiMAX supports handover so that the MS can receive the seamless data service while mobile. Specifically, seamless service is maintained when the MS migrates from existing Base Station (BS) coverage to coverage of another BS. Before handover, the MS obtains information on its neighboring BSs to determine which BS coverage to move to. A process for obtaining the neighboring BS information is referred to as scanning. Since the MS should receive the continuous data service during the scanning, the scanning and the data service need to be carried out without affecting each other.
Research has been conducted in order to provide both point-to-point telephone communication and various additional services. For example, group communication, specifically, a Push To Talk (PTT) service, using a broadband wireless access system, such as WiMAX, is one such additional service. PTT group communication services a plurality of MSs so that they receive the same data. When the same data is unicast to the MSs, radio resources are greatly wasted. Therefore, a multicasting method for transmitting the same data to the plurality of the MSs at the same time is suitable for use with PTT group communication services.
An MS using the unicast service can accomplish both scanning and data reception without having them affect each other by synchronizing a scanning interval and a unicast data reception interval. FIG. 1 is a diagram illustrating the scanning interval and the data reception interval. A scanning cycle 110 includes a scanning interval 114 and a scheduling interval 116, as shown in FIG. 1. The scheduling interval 116 is the unicast data reception interval. Specifically, the scanning cycle 110 starts from a frame indicated by a scan offset 112, and the scanning interval 114 is positioned at the front of the scanning cycle 110.
A multicast service transmits data to a plurality of MSs at the same time. When the scanning intervals of the MSs are different from each other, the scanning interval and a multicast data reception interval can overlap. As a result, when scanning is conducted in the multicast data reception interval or data is received in the scanning interval, multicast data reception quality or handover success rate can be degraded. Additionally, when the MS in an awake mode receives both of the unicast service and the multicast service, the reception intervals of the unicast data and the multicast data do not match. As a result, the unicast data and the multicast data affect each other and their reception quality can be degraded.